1. Field of the Invention
This invention relates to the field of thermal printing apparatus and, more particularly, to method and apparatus for preventing unevenness in printing depth in thermal printers.
2. Description of the Prior Art
Thermal printing apparatus is popularly used in facsimile receivers or other printers mainly because they are small, light, and easy to maintain. The thermal printing receivers used in facsimile apparatus and other printers have a thermal printing head which includes a line of heat generating elements, shift registers and latch circuitry. A line of data, each bit of which corresponds to each one of the heat generating elements, is supplied to the head along with a print enabling signal. Electric current flows through preselected heat generating elements, i.e., elements which receive a print signal, when the print enabling signal is active. The current receiving elements then generate heat which causes a portion of a thermosensitive recording medium in contact with the elements to darken. Lines of print data are successively supplied to the head, along with print enabling signals, until the printing operation is completed.
As with all other types of printing apparatus, there exists a need for thermal printing apparatus to print as rapidly as possible. One method for increasing the printing speed of thermal printing apparatus is to reduce the driving cycle. However, it has been noted that by shortening the driving cycle unevenness in printing depth normally occurs. Because printing the current line begins at a time when heat due to printing the previous line remains, temperature differences exist between previously heated elements and previusly non-heated elements. The previously heated elements reach each temperature level sooner than previously non-heated elements. This results in unevenness in printing depth.
Conventional methods for preventing unevenness in printing depth modulate the driving current of the heat generating elements in response to their previous print status. Typically an element which was printing in the previous line will be driven with less current than an element which did not print the previous line. These methods rely upon a constant time interval between consecutively printed lines to provide uniformity in printing depth. However, the time interval between consecutively printed lines is not constant. Therefore, these methods result in non-uniform printing depth between consecutively printed lines. Furthermore, since the time interval between consecutively printed lines is not constant, and since the temperature decay of the heat generating elements is not linear, unevenness in printing depth may occur between elements of the same line.
Also, in the above described methods there is a need to provide the printing head with a plurality of additional elements. As an example, some apparatus which implement the above described method require at least two lines of registers. Other apparatus for implementing the above methods require logic circuits, corresponding to each element, for transforming the current data in accordance with the presence or absence of previous data. Many devices for implementing the above methods use enabling signals with different active periods to modulate the driving current to the heat generating element. These types of methods require logic circuitry to select between the several enabling signals. Other devices for controlling the magnitude of electric current supplied to the heat generating elements require at least two switches for selecting either one of at least two sources of electric current, each having a different magnitude. Accordingly, the above described methods require a large number of logic circuits to modulate the current supplied to the heat generating elements.